Many copying machines, printers, and the like based on an electrophotographic method use, as thermal fixing methods, a heating roller fusing method of a contact heating type which exhibits high thermal efficiency and safety and a film heating method of an energy saving type.
As shown in FIG. 19, a thermal fixing device based on the heating roller fusing method is basically comprised of a heating roller (fixing roller) 40 serving as a heating rotating member containing a heater 41 such as a halogen heater and an elastic pressurizing roller 50 serving as a pressurizing rotating member which comes into contact with the heating roller 40 to pressurize it. This pair of rollers are rotated, and a printing material P (a transfer sheet, print sheet, electrostatic printing sheet, electrofax sheet, or the like) serving as a material to be heated on which an unfused toner image is formed/borne is guided to the nip area (fusing nip area) between the roller pair. The printing material is then passed through the nip area to thermal-fuse the unfused toner image as a permanent fixed image on the printing material surface by using heat from the heating roller 40 and pressurizing force at the nip area. The heating roller 40 is configured such that a separating layer 43 made of fluororesin or the like is formed on the outer surface of a hollow cored bar 42 made of iron or the like. The pressurizing roller 50 is configured such that an elastic layer 52 made of silicone rubber or the like is formed on the surface of a cored bar 51 made of iron or the like, and a separating layer 53 such as a fluororesin tube is formed on the outer surface of the elastic layer. The heating roller 40 is heated by energizing the heater 41. The surface temperature of the heating roller 40 is detected by a temperature detection element such as a thermistor to be maintained at a predetermined temperature, thereby heating the nip area.
Thermal fixing devices based on the film heating method (on-demand fusing devices) are disclosed in, for example, Japanese Patent Laid-Open Nos. 63-313182, 2-157878, 4-44075, and 4-204980. FIG. 20 shows a typical example of these devices. Referring to FIG. 20, reference numeral 60 denotes a film assembly, which is configured such that a heater 61 having an electro heat-producing resistance layer formed on a ceramic substrate made of alumina, aluminum nitride, or the like is fixed to a stay holder 62 made of a heatproof resin, and a heatproof thin film 63 (to be referred to as a fusing film hereinafter) made of a resin such as polyimide or a metal such as SUS is loosely fitted on the stay holder 62. The heater 61 of the film assembly 60 and a pressurizing roller 50 clamp and pressurize the fusing film 63 to form a fusing nip area.
The fusing film 63 is conveyed/moved in the direction indicated by the arrow, while being in tight contact with and slid on the heater 61 at the fusing nip area, by the rotating/driving force of the elastic pressurizing roller 50 in the direction indicated by the arrow. The elastic pressurizing roller 50 is obtained by forming an elastic layer 52 made of silicone rubber or the like and a separating layer 53 made of fluororesin or the like on the surface of a cored bar 51. The temperature of the heater 61 is detected by a temperature detection means 64 such as a thermistor placed on the back of the heater and fed back to an energization control unit (not shown) to adjust the temperature of the heater 61 to a predetermined constant temperature (fusing temperature).
Various types of image forming apparatuses such as printers and copying machines which use such a thermal fixing device based on the film heating method have many advantages over image processing apparatuses using a thermal fixing device based on the conventional heating roller method. For example, they can eliminate the necessity of pre-heating and shorten the wait time because of high heating efficiency and quick startup.
Currently, a wide variety of printing materials used for image formation, which greatly change in thickness and surface properties, have been on the market. It is known that in the above conventional thermal fixing device, the fusing properties of toner images on such printing materials are influenced by the thicknesses and surface properties of the printing materials. The fusing properties are considerably degraded on a type of paper with rough surface properties, in particular. This is because the contact area between the heating member and the printing material decreases in the fusing nip area, and a sufficient amount of heat is not supplied to the toner on the printing material. As a consequence, in order to obtain good fusing properties even on a type of paper with poor surface properties, it is necessary to increase the fusing pressurizing force or fusing temperature.
The method of increasing the fusing pressurizing force, however, leads to an increase in the driving torque of the fusing device and hence tends to increase the device cost. In the above thermal fixing device based on the film system, in particular, since a fusing film that is a heating rotating member is slid on the heater serving as a heating member at the fusing nip area, the rotational torque tends to be high. This makes it difficult to increase the pressurizing force; the total pressure is limited to about 196 N (20 kgf) at most, and the linear pressure in the fusing nip area is set to be relatively low. It is therefore inevitable to raise the fusing temperature in order to improve the fusing properties of a type of paper with poor surface properties.
If, however, the fusing temperature is simply raised, an excessive amount of heat is supplied to a thin sheet or a sheet with good surface properties. This may lead to adverse effects, e.g., the occurrence of hot offset and an increase in the amount of curl of a sheet.
In addition, not only a fusing temperature but also a fusing nip width is important parameters for contradictory phenomena such as the fusing properties of toner images on printing materials, the curls of printing materials, and the hot offset of toner. That is, as the fusing nip width is increased, the time during which heat is transferred to a printing material is prolonged even at a low fusing temperature, and hence good fusing properties may be realized. In contrast, this suppresses the occurrence of phenomena such as the curls of printing sheets and the hot offset of toner.
Although the fusing nip width mainly depends on the hardness of a pressurizing roller and the pressurizing force of a pressurized spring, they change to some extent. Different fusing devices therefore have different fusing nip widths. For this reason, if fusing temperature setting is made in consideration of variations in fusing nip width, it is very difficult to satisfy requirements for all the phenomena such as fusing properties, curl, and hot offset with respect to various types of printing materials described above by setting only one temperature.
As described above, it is difficult to set fusing conditions optimal for both a printing material with rough surface properties and a printing material with good smoothness. Conventionally, in order to cope with this problem, a user selects a fusing temperature in accordance with the printing material to be used. It is, however, difficult for the user to set a fusing mode in accordance with a parameter like surface roughness that is incomprehensible to the user. For this reason, it has been required to automatically set an optimal fusing temperature in accordance with the printing material to be used (surface roughness in particular).
From this viewpoint, a method of feeding back information for fusing control by detecting the temperature of a printing material delivered from a fusing nip is disclosed in, for example, Japanese Patent Laid-Open Nos. 1-150185, 6-308854, 7-230231, and 2002-214961.
FIG. 21 shows an example of a conventional thermal fixing device designed to perform temperature detection by using a contact type sensor. In this thermal fixing device, a temperature sensor 18 such as a temperature detection thermistor is placed downstream of the fusing nip, and a facing member such as a rubber roller is placed at a position to face the temperature sensor 18 to clamp a printing material between them and measure its temperature.
FIG. 22 shows an example of a conventional thermal fixing device designed to perform temperature detection by using a non-contact type sensor. In this thermal fixing device, a non-contact type sensor 20 such as an infrared sensor is placed downstream of the fusing nip to measure the temperature of a printing material in a non-contact manner.
There has been proposed a method of preventing curl and hot offset by lowering the fusing temperature for a thin sheet or smooth sheet, which is easily heated, by feeding back information for energization control on the heater of a heating member on the basis of the measurement result on the delivery temperature of such a printing material, while satisfying requirements for fusing properties by raising the fusing temperature for a printing material with rough surface properties or a thick printing material.
The following problems, however, have arisen in the conventional thermal fixing device described above.
A method of detecting the temperature of a printing material while clamping it between a temperature sensor and a facing member such as a roller as shown in FIG. 21 will be described first. In this method, since the facing member of the temperature sensor is always in contact with a printing material, heat in the printing material is taken away by the facing member. This makes it impossible to accurately detect the temperature of the printing material. In order to stably clamp and convey a printing material, a rubber roller serving as a facing member needs to be formed to have a certain size. The heat capacity of the roller as the facing member cannot be neglected, and hence it is difficult to make a noticeable difference in the temperature detection thermistor in accordance with the surface roughness or thickness of a printing material. In addition, if a rubber roller is used as a facing member, the amount of heat taken away from a recoding material by the rubber roller changes depending on the surface properties of the rubber roller. If the surface condition of the rubber roller has changed after the passage of sheets, this causes a variation in detected temperature.
In addition, when the temperature of a printing material delivered from the fusing nip, consideration must be given to the influence of the dissipation of heat from the printing material delivered from the fusing nip. This is because the state of dissipation of heat from the printing material changes due to the influences of convection in the device and the temperature outside of the device in the area where the printing material delivered from the fusing nip is conveyed. For this reason, temperature detection is influenced more easily with the lapse of time after a printing material is delivered from the fusing nip, and hence different detection results are obtained by the temperature detection element even if identical printing materials are conveyed. For this reason, if the temperature detection element is placed next to the fusing nip, the influences of convection in the device and the like can be reduced, and the accuracy of the discrimination of a printing material can be improved.
On the other hand, if a temperature detection element such as a thermistor is placed next to the fusing nip, since the temperature of an atmosphere in which the temperature detection element changes depending on the heated state of the fusing roller and the fusing member for a fusing film or a past thermal fixing history, the type of printing material must be properly discriminated in each case.
From the above viewpoint as well, when a member such as a rubber roller having a large heat capacity is placed next to the fusing nip to face and come into contact with the temperature detection element, the rubber roller having a large heat capacity is easily influenced by the ambient temperature next to the fusing nip, and the heated state of the rubber roller placed to face the temperature detection element greatly changes. As a consequence, the amount of heat taken away from a printing material delivered from the fusing nip by the rubber roller changes. This makes it difficult to accurately discriminate the type of printing material in every case.
According to the prior art, in particular, a printing material is discriminated with a predetermined value by the temperature detection element placed next to the fusing nip. If, however, the temperature detection element is actually placed next to the fusing nip, the influence of ambient temperature cannot be neglected. This makes it difficult to accurately discriminate the type of printing material and optimally control the thermal fixing device.
A problem in temperature detection using a non-contact type sensor like the one shown in FIG. 22 will be described next. When a printing material is heated/fused, since moisture contained in the printing material is also heated at the same time, steam is produced from the surface of the printing material. The surface of the non-contact sensor is then covered with the steam. As a consequence, the sensor cannot correctly detect the temperature of the printing material. As described above, the temperature of a printing material is preferably detected immediately after it is delivered from the fusing nip in terms of reducing the influence of dissipation of heat from the printing material immediately after it is delivered from the fusing nip. On the other hand, temperature detection is most susceptible to the influence of steam from the printing material immediately after it is delivered from the fusing nip.
In order to prevent this, it may be conceivable to prevent the surface of a non-contact type sensor from being covered with steam by forming an air path or the like using a fan. In this case, however, this air path influences the temperature of the surface of a printing material. In consideration of variations in wind velocity, it is difficult to discriminate the printing material.
As described above, in practice, no technique has been developed to discriminate the type of printing material by measuring the temperature of the printing material using a non-contact type sensor such as an infrared sensor.